Detection systems, such as ion mobility spectrometers (IMS), mass-spectroscopy, and other detection systems, can use added substance to enhance the detection of target analyte substances. The added substances can be dopants, also know as marker compounds, for example. These will generally be referred to as “dopants.” Examples of a detection system using dopants can be found in EP1649274A, PCT/GB06/001739, W006/129101, W007/010,261, PCT/GB06/003677 and PCT/GB07/002,160, each of which is hereby incorporated by references.
In some applications, it is preferable to use one or more dopants at a very low level in order to facilitate a fast removal of the dopants when desired. In some applications, the dopant is added only in particular circumstances, such as when the presence of a particular analyte substance is suspected but not clearly determined. However, it is difficult to add a dopant rapidly, and furthermore, it can take considerable amount of time to remove the dopant when it is no longer needed. Thus, there is a need for improved methods of delivering and removing dopants from detection systems.